Lightning strike protection

ABSTRACT

An aircraft component having an external surface includes an improved lightning strike protection surface film disposed on or proximate to the external surface. The surface film includes a preform that includes a substrate having a first areal weight density and a plurality of spaced carbon nanotubes grown on the substrate, the nanotubes having a second areal weight density. The sum of the first areal weight density and the second areal weight density is less than about 500 grams per square meter, and the preform has a surface resistance less than about 1 ohm/square.

TECHNICAL FIELD

The technical field relates to aircraft and aircraft components, andmore particularly relates to an improved lightning strike protectionsurface film for composite aircraft components and other compositestructures.

BACKGROUND

The outer surfaces of aircraft components such as fuselages, wings, tailfins, engine nacelles, and the like, are typically constructed fromnon-metal composite materials, aluminum, or hybrid materials thatinclude a combination of composite materials and metal. When lightningstrikes a metal outer skin of an aircraft, the metal skin provides ahighly conductive path that permits an electrical current to pass acrossthe metal skin from a lightning strike point to a lightning exit pointwithout substantial damage to the surface of the aircraft. Many modernaircraft components such as engine nacelles, however, are constructed ofstrong but light-weight composite materials that help to minimize theoverall weight of the aircraft. These composite materials often comprisecarbon or graphite reinforcement fibers distributed within a polymericmatrix. Such composite structures typically are substantially lesselectrically conductive than metal structures, and are substantiallyless capable of distributing and dissipating electrical energy resultingfrom a lightning strike. Accordingly, external surfaces of suchcomposite aircraft components often include lightning strike protectionthat provides a highly conductive electrical path along their externalsurfaces. Such a conductive path permits the electrical energyassociated with a lightning strike to be rapidly dissipated across theprotected surface, which helps minimize damage to the surface of theaircraft component at the lightning strike point.

Airworthiness certification authorities have established standards forlightning strike protection for various types of aircraft and aircraftcomponents. Based upon the probability of a lightning strike to aparticular portion of an aircraft and the probable intensity of theelectrical energy generated by such a strike, authorities havedesignated various potential strike zones for each type of aircraft andthe probable current waveforms that structures and systems within eachzone must withstand without substantial damage. Authorities designatethese different strike zones as Zones 1A and 1B, Zones 2A and 2B, andZone 3. These various strike zone designations are described in U.S.Pat. No. 5,417,385 and SAE ARP 5414, for example, and are understood bypersons skilled in the art.

Composite aircraft components which are classified as Zone 1A requirethe greatest degree of lightning strike protection. SAE ARP 5416 setsforth lightning strike test procedures for certifying Zone 1A aircraftcomponents. In order to satisfy the requirements of SAE ARP 5416, a testpanel that replicates the structure of the Zone 1A component mustwithstand an artificially produced lightning strike having a specifiedcurrent wave form without penetration through the test panel.

Current lightning strike protection systems for non-metal compositeaircraft structures typically comprise a lightning strike protectionsurface film that includes a metal foil or mesh that is disposed on orproximate to an external surface of the composite structure tofacilitate the distribution and dissipation of electrical energygenerated by a lightning strike on the protected surface. For example, ametal foil or mesh can be embedded within a thin layer of a polymericmaterial that is disposed on a surface of a composite structure. Oneprocess for bonding a metal foil or mesh on a surface of a laminatedcomposite structure for lightning strike protection is described in U.S.Pat. No. 5,470,413, for example. Alternatively, a metal foil or mesh canbe incorporated into a surface portion of a laminated compositestructure as the structure is fabricated. For example, U.S. Pat. No.5,417,385 describes fabricating a laminated composite structure with ametal foil or mesh disposed proximate to its outer surface. In order toprovide a smooth and aerodynamic outer surface for painting, a thinpolymeric surface layer can be provided over the surface film containingthe metal foil or mesh.

One common type of lightning strike protection includes an aluminum foilor mesh that is disposed on or proximate to the external surface of aprotected composite structure. In one example, an aluminum foil or meshthat is capable of satisfactorily protecting a Zone 1A component has anareal weight density of about 0.02 pounds per square foot or about 74grams per square meter (gsm). The term “areal weight density” iscommonly associated with thin materials such as fabric, tape, foils andthe like, and is well known among persons skilled in the art. As usedherein, “areal weight density” refers to the weight of the materialdivided by its area (for example, length times width of a rectangularpiece) of the material. The total areal weight density of conventionallightning strike protection systems (aluminum foil or mesh, polymermatrix, and a fiberglass corrosion isolation layer) can be about 0.11pounds per square foot or about 500 gsm, or less.

Though an aluminum foil or mesh like that described above has proven tobe effective for lightning strike protection for Zone 1A components,such a metal foil or mesh can add undesired weight to an aircraft. Inaddition, differences in the coefficients of thermal expansion (CTEs)between the metal foil or mesh and the polymers and reinforcementmaterials to which it is attached can introduce thermal stresses in theindividual constituents. As a result, the protected surface can becomeprone to microcracking when subjected to repeated variations in ambienttemperature routinely experienced by aircraft during service. At highaltitudes, an aircraft (including its external components) is oftenexposed to relatively low ambient temperatures, whereas on the ground,the aircraft is exposed to relatively high ambient temperatures. Thesecyclic variations in temperature can be substantial. When a metal foilor mesh and the surrounding polymeric material have different CTEs, suchvariations in temperature can induce differential thermal expansionbetween the metal and the associated composite structure, and theresulting thermal stresses can cause microcracks to form in the surfaceof the composite structure. Such microcracks are undesirable as they canpermit the ingress of moisture or chemicals into the composite structureand cause the structure to degrade.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates carbon nanotubes grown on a substrate according tovarious embodiments.

Aircraft manufacturers and their suppliers are continually looking forways to reduce the weight of their product for a number of reasons,including greater range per unit of fuel and/or greater fuel efficiency.Accordingly, there is a need for an improved surface film for lightningstrike protection for Zone 1A aircraft components that is lighter inareal weight density than known surface films that include metal foilsor screens. In particular, there is a need for an improved surface filmfor lightning strike protection of Zone 1A components that has an arealweight density less than about 500 gsm. In addition, there is a need forimproved durability of the surface film for Zone 1A lightning strikeprotection, and for one that is less likely to be prone to surfacemicrocracking than surface films that include metal foils or screens.

SUMMARY OF THE INVENTION

In one embodiment, an aircraft component having an external surfaceincludes a lightning strike protection surface film disposed on orproximate to the external surface. The surface film can include preformin a polymer matrix. The preform includes a substrate having a firstareal weight density and a plurality of spaced carbon nanotubes grown onthe substrate, the nanotubes having a second areal weight density. Thesum of the first areal weight density and the second areal weightdensity can be less than about 500 gsm, and the preform can have asurface resistance less than about 1 ohm/square.

In another embodiment, a lightning strike protection surface filmincludes a substrate having a first areal weight density and a pluralityof spaced carbon nanotubes grown on the substrate, the nanotubes havinga second areal weight density. The sum of the first areal weight densityand the second areal weight density can be less than about 500 gsm, andthe substrate with grown-on nanotubes can have a surface resistivityless than about 1 ohm/square.

Another embodiment of the invention includes a method of producing anaircraft component with lightning strike protection. The method caninclude providing an aircraft component having an outer surface portion,forming a lightning strike protection surface film by providing asubstrate having a first areal weight density and growing on thesubstrate a plurality of spaced carbon nanotubes having a second arealweight density, and bonding the lightning strike protection surface filmto the outer surface portion to form the aircraft component withlightning strike protection. The method can further include selectingthe composition and first areal weight density of the substrate andcontrolling the second areal weight density of the carbon nanotubes suchthat the sum of the first areal weight density and the second arealweight density is less than about 500 gsm, and such that the substratewith grown-on nanotubes has a surface resistivity less than about 1ohm/square.

An additional embodiment includes a composite aircraft component havingat least one layer comprising a woven or non-woven substrate and aplurality of carbon nanotubes grown on the substrate. The substrate withgrown-on carbon nanotubes can provide structural reinforcement to thecomposite aircraft component, and in addition or alternatively, canprovide at least some level of lightning strike protection.

An additional embodiment includes a fiber reinforced composite surfacefilm on the aircraft component comprising carbon fibers in a resinmatrix. The carbon fibers in the said surface film have a length greaterthan at least about 6 mm and include carbon nanotubes grown on thefibers, and the nanotubes are substantially uniformly distributed on thefibers and have a length between about 1 and 100 microns.

These and other aspects and features of the invention will be understoodfrom a reading of the following detailed description.

DETAILED DESCRIPTION

An improved surface film for lightning strike protection of Zone 1A,Zone 2A and 2B, and Zone 3 aircraft components is described below, andhas a lighter areal weight density than conventional surface films thatinclude metal foils or meshes. Such an improved surface film is alsoless likely to be prone to surface microcracks than a surface film thatincludes a metal foil or mesh.

In one embodiment, a surface film for lightning strike protection isdisposed on or proximate to an external surface of an aircraftcomponent. As used herein, the phrase “proximate to” means at or near asurface, wherein a film disposed proximate to a surface is located at ornear the surface. In one embodiment, an electrically conductive surfacefilm is not more than about 0.5 millimeter (mm) from the externalsurface of the structure. The surface film can include a substratehaving a plurality of carbon nanotubes (“CNTs”) grown on the substrate.The surface film can include a substrate having a plurality of carbonnanotubes grown on the substrate, where the carbon nanotubes can besingle wall carbon nanotubes (SWCNTs), double wall carbon nanotubes(DWCNTs), multiwall carbon nanotubes (MWCNTs), or any combinationthereof. Alternatively, the carbon nanotubes may be substituted by, orcombined with, carbon nanofibers (CNFs). Hereinafter the terms “CNTs”and “carbon nanotubes” are meant to include carbon nanotubes, carbonnanofibers and combinations of carbon nanotubes and carbon nanofibers,and the terms “grown-on CNTs” and “grown-on carbon nanotubes” are meantto include carbon nanotubes, carbon nanofibers and combinations ofcarbon nanotubes and carbon nanofibers grown on the substrate.Preferably, the substrate is constructed of materials that haverelatively low electrical resistivities. Alternatively, substrates thathave relatively high electrical resistivities may be used in certainapplications where lesser degrees of lightning strike protection areadequate. The substrate and grown-on CNTs combine to form asubstantially flexible and electrically conductive preform. As usedherein, the term “preform” refers to a substrate with a plurality ofCNTs grown on the substrate.

The CNTs can be grown on the substrate using a process like thatdescribed in International Application No. PCT/US2007/011577, filed May15, 2007, for example, or by a process like that described in“Electrical Properties of Hybrid Woven Composites Reinforced withAligned Carbon Nanotubes” by N. Yamamoto et al., 49^(th)AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and MaterialsConference, Apr. 7-10, 2008, Schaumburg, Ill. (“Yamamoto”), for example.The disclosures of International Application No. PCT/US2007/011577 andYamamoto are hereby incorporated by reference in their entireties. Asdescribed in PCT/US2007/011577, one method of growing CNTs on thesubstrate can include functionalizing the surface of the substrate byexposing the surface to an oxidizing gas, and then forming catalysts onthe surface of the substrate by immersing the substrate in a catalystsolution. Referring to FIG. 1, for example, nanofibers 100 are shown oncarbon fiber substrates which were surface treated by gas oxidation. Asshown, thick and aligned carbon nanotube structures were observed on thecarbon fiber substrates. Alternatively, catalysts can be formed on thesurface by subjecting the substrate to electrodeposition. Chemical vapordeposition can then be used to facilitate the growth of the CNTs on thesurface of the substrate. When electrodeposition is used to form thecatalysts on the substrate, the process can include a reductant such assodium hypophosphite, for example. The oxidizing gas can be selectedfrom ozone, carbon dioxide, and mixtures thereof, for example. Thesubstrate can be exposed to the oxidizing gas at a temperature ofbetween about 100° C. and 900° C. Where the oxidizing gas comprisesozone, the substrate can be exposed at a temperature of between about100° C. and about 200° C., and where the oxidizing gas comprises carbondioxide, the substrate can be exposed at a temperature of between about400° C. and about 900° C. The catalyst solution can include a water oralcohol solution and soluble salts selected from salts of iron,molybdenum, nickel, cobalt, and combinations thereof, for example. Thesubstrate can be dried after immersing the substrate in the solution andbefore subjecting the substrate to chemical vapor deposition to form theCNTs. Chemical vapor deposition can take place at a temperature betweenabout 600° C. and about 900° C., and can utilize a hydrocarbon gasesselected from acetylene, ethylene, methane, and combinations thereof.The areal weight density of the CNTs grown on the substrate can beclosely controlled by controlling the reaction time during chemicalvapor deposition. The aforementioned process yields a preform comprisinga substrate with grown-on CNTs that are substantially uniformlydistributed over the external surfaces of the substrate such that thegrown-on CNTs form a substantially continuous network of CNTs that iscoextensive with the substrate. In one embodiment, a substantial portionof the grown-on CNTs touch or are within about 5 microns of at least oneother grown-on CNT. For example, at least about 75 percent of thegrown-on CNTs can touch or be within about 5 microns of at least oneother grown-on CNT.

Each of the grown-on CNTs can include a first end that, forsubstantially each CNT is attached to at least a portion of thesubstrate, and an opposed second end that generally extends away fromthe first end and the substrate. The CNTs can be generally straight orcan have a generally helical shape or another shape. The lengths of thegrown-on CNTs can be from about 5 microns to about 100 microns, or morepreferably, can be between about 10 microns and about 40 microns, andthe diameters of the grown-on CNTs can be from about 1 nanometer (nm) toabout 200 nm. The morphology of the grown-on CNTs can vary from bulkyand entangled, to loose bundles, to random and helical. In oneembodiment, the preform comprising the substrate with grown-on CNTs issufficiently flexible to conform to a curved surface like that commonlyfound on exterior surfaces of an aircraft (including its variouscomponents). In another embodiment, a thin layer of an electricallyconductive metal such as nickel, copper, gold, silver, platinum or thelike can be deposited on the substrate, or alternatively, on theperform, i.e., both the substrate and the grown-on CNTs, to reduce thestructure's electrical resistivity, such as by electroplating, physicalvapor deposition, or the like. The thickness of the deposited metallayer can be from about 0.001 microns to about 50 microns, for example.

In one embodiment, the preform can be embedded within a polymeric resinto form a lightning strike protection surface film. When cured, thepolymeric resin binds the preform constituents, namely the substrate andthe grown-on CNTs, in a fixed position on or proximate to a surface of acomponent or structure. In one embodiment, the preform can beimpregnated with an epoxy or thermoplastic resin of a type commonly usedto fabricate composite aircraft structures, and the resulting surfacefilm can be incorporated on or adjacent to the surface of a compositestructure during lay up of the composite structure using fabricationmethods known in the art. In one embodiment, the preform is positionedwithin about 0.5 mm of the protected external surface of the compositestructure. If the external surface of the composite structure is to bepainted in order to provide the surface with a smooth and aestheticallypleasing appearance, the preform can be located within about 0.5 mm (orless) of the surface before paint is applied to the surface. The surfacefilm can be cured together with other portions of the compositestructure using known methods such that the surface film is disposed onor proximate to an external surface of the cured structure. In anotherembodiment, the preform can be infused or impregnated with a polymericresin, the resin can be cured to form a durable sheet or film, and thesheet or film can be bonded onto an external surface of an aircraftcomponent for lightning strike protection. For example, the preform canbe embedded within a polyurethane film, and the resulting flexiblesurface film can be bonded to an external surface of a compositestructure with an adhesive or the like. After bonding, the preform islocated proximate to an outermost surface of the structure. In oneembodiment, the preform can be sized with an epoxy before embedding thepreform within a polymeric film. The epoxy sizing helps keep-thegrown-on CNTs attached to the substrate during shipment or handlingand/or promotes bonding during fabrication.

In one embodiment, the areal weight density of the substrate and theareal weight density of the grown-on CNTs is selected and controlledsuch that the areal weight density of the preform, that is the combinedareal weight densities of the substrate and the grown-on CNTs, is lessthan or equal to the combined areal weight density of a metal foil ormesh and its associated fiberglass isolation layer, and is capable ofproviding lightning strike protection for a Zone 1A aircraft component.For example, when compared to an aluminum foil or mesh and an associatedfiberglass isolation layer having a combined areal weight density ofabout 500 gsm, the combined areal weight densities of the substrate andthe grown-on CNTs can be limited or controlled to be less than about 500gsm. Specific areal weight densities for various embodiments thatinclude different preforms are discussed below. Test results suggest acorrelation between the resistivity of surface films made in accordancewith the invention and the lightning strike criteria of the SAE ARP 5416specification. A surface film that includes a preform having a surfaceresistivity less than about 1 ohm/square correlates with the Zone 2Atest requirements of SAE ARP 5416, and a surface film in accordance withthe invention and having a surface resistivity less than about 0.5ohm/square may meet the Zone 1A test requirements of SAE ARP 5416. Asused herein, surface resistivity is the surface resistivity measuredusing a four point probe method, as is known in the art, such as themethod set forth in ASTM F390—Standard Test Method for Sheet Resistanceof Thin Metallic Films with a Collinear Four-Probe Array, for example(hereinafter “the four point probe method”). As discussed in detailbelow, the invention includes various embodiments of lightning strikeprotection surface films that include preforms that may be capable ofproviding Zone 1A or Zone 2A lightning strike protection for aircraftcomponents, and have areal weight densities that are less than the arealweight densities of comparable surface films that include metal foils orscreens (i.e. less than about 500 gsm).

In one embodiment, the substrate can be a braided fabric, woven fabric,or non-crimp fabric constructed of elongated yarns formed ofelectrically conductive fibers. The structure of the fabric substratecan be substantially similar to braided, woven or non-crimp fabriccommonly used as reinforcements in composite aircraft structures, forexample. The electrically conductive fibers can be carbon fibers (suchas standard modulus carbon fibers, high modulus carbon fibers, heattreated carbon fibers, metal coated carbon fibers, and the like), or canbe CNT reinforced polyacrylonitrile (PAN) carbonized fibers (CNTs withinthe PAN fibers). The invention is not limited to carbon fibers, and canbe applied to other electrically conductive fibers known to thoseskilled in the art, for example silicon carbide fibers. The areal weightdensity of the braided, woven or non-crimp fabric substrate can be about70 gsm to about 400 gsm. A plurality of CNTs can be grown on thesubstrate as described above, and can have an areal weight density ofabout 2 gsm to about 100 gsm. The combined areal weight densities of thebraided, woven or non-crimp fabric substrate and the grown-on CNTs canbe less than about 500 gsm. The substrate and the grown-on CNTs providean electrically conductive preform for use in providing lightning strikeprotection to an external surface of a composite structure, such as acomposite aircraft structure. When used for Zone 1A lightning strikeprotection, one embodiment of a preform comprising the braided, woven ornon-crimp fabric substrate and the grown-on CNTs can have a surfaceresistivity less than about 0.5 ohm/square when measured by thefour-point probe method described above. When used for Zone 2A lightningstrike protection, for example, the preform can have a surfaceresistivity less than about 1 ohm/square.

In another embodiment, the substrate can be a braided, woven ornon-crimp fabric constructed of elongated yarns or fibers formed of afirst plurality of CNTs, such as by wet, dry, melt, gel orelectro-spinning, for example. Preferably, the first plurality of CNTsthat form the fibers includes CNTs having lengths greater than about 6mm. In this embodiment, the areal weight density of the braided, wovenor non-crimp fabric substrate can be about 15 gsm to about 200 gsm. Asecond plurality of CNTs can be grown on the substrate as describedabove, and can have an areal weight density of about 2 gsm to about 100gsm. In this embodiment, the combined areal weight densities of thewoven/braided substrate and the grown-on CNTs can be less than about 300gsm. When used for Zone 1A lightning strike protection, one embodimentof the preform comprising the braided, woven or non-crimp fabricsubstrate and the grown-on CNTs can have a surface resistivity less thanabout 0.5 ohm/square when measured by the four-point probe method. Whenused for Zone 2A lightning strike protection, for example, the preformcan have a surface resistivity less than about 1 ohm/square.

Another embodiment includes a non-woven mat, scrim or veil constructedof electrically conductive fibers as the substrate. The substrate caninclude either continuous fiber strands or chopped fiber strands. Whenthe substrate includes chopped fiber strands, the fibers preferably havelengths greater than about 6 mm. The electrically conductive fibers canbe carbon fibers (such as standard modulus carbon fibers, high moduluscarbon fibers, heat treated carbon fibers, metal coated carbon fibers,and the like), or can be CNT reinforced polyacrylonitrile (PAN)carbonized fibers (CNTs within the PAN fibers), silicon carbide fibersor other electrically conductive fibers. The areal weight density of thesubstrate is preferably between about 70 gsm to about 400 gsm. Aplurality of CNTs can be grown on the substrate as described above, andcan have an areal weight density of about 2 gsm to about 100 gsm. Theareal weight densities of the preform or the combined areal weightdensities of the substrate and the grown-on CNTs, can be less than about500 gsm. When used for Zone 1A lightning strike protection, oneembodiment of a preform comprising the substrate and the grown-on CNTscan have a surface resistivity less than about 0.5 ohm/square whenmeasured using the four-point probe method. When used for Zone 2Alightning strike protection, for example, the preform can have a surfaceresistivity less than about 1 ohm/square.

In another embodiment, the substrate includes a non-woven mat, veil orscrim containing fibers formed of a first plurality of CNTs, such as bywet, dry, melt, gel or electro-spinning, for example. Preferably, thefirst plurality of CNTs that form the fibers includes CNTs havinglengths greater than about 6 mm. In this embodiment, the areal weightdensity of the mat, veil or scrim substrate can be about 15 gsm to about200 gsm. Optionally, a second plurality of CNTs may be grown on thesubstrate as described above, and can have an areal weight density ofabout 2 gsm to about 100 gsm. In this embodiment, the combined arealweight densities of the mat or scrim substrate and the grown-on CNTs canbe less than about 300 gsm. When used for Zone 1A lightning strikeprotection, one embodiment of the preform comprising the mat or scrimsubstrate, with or without the grown-on CNTs, can have a surfaceresistivity less than about 0.5 ohm/square when measured using thefour-point probe method. When used for Zone 2A lightning strikeprotection, for example, the preform can have a surface resistivity lessthan about 1 ohm/square.

Another embodiment includes a CNT paper as the substrate. Preferably, afirst plurality of CNTs forming the paper substrate includes CNTs havinglengths greater than about 6 mm. In this embodiment, the areal weightdensity of the CNT paper substrate can be about 15 gsm to about 200 gsm.Optionally, a second plurality of CNTs may be grown on the CNT papersubstrate as described above, and can have an areal weight density ofabout 2 gsm to about 100 gsm. In this embodiment, the combined arealweight densities of the CNT paper substrate and the grown-on CNTs can beless than about 300 gsm. When used for Zone 1A lightning strikeprotection, one embodiment of a preform comprising the CNT papersubstrate, with or without the grown-on CNTs, can have a surfaceresistivity less than about 0.5 ohm/square when measured using thefour-point probe method. When used for Zone 2A lightning strikeprotection, for example, the preform can have a surface resistivity lessthan about 1 ohm/square.

Though the substrates and the grown-on CNTs as described above areinherently electrically conductive, to satisfy the requirements ofcertain lightning strike applications, the preform can be subjected to aheat-treatment step to obtain further improvement in electricalconductivity. The preform can be heat-treated in a vacuum or in an inertatmosphere, for example argon, nitrogen, and the like, and attemperatures of between about 600° C. and 1,800° C. for times rangingfrom about 0.25 to 24 hours. When used for Zone 1A lightning strikeprotection, a preform after the heat-treatment step can have a surfaceresistivity less than about 0.5 ohm/square when measured using thefour-point probe method. When used for Zone 2A lightning strikeprotection, for example, the preform can have a surface resistivity lessthan about 1 ohm/square.

Though the substrates described above are constructed of materials thatare highly electrically conductive, for certain applications, alightning protection surface film according to the invention can alsoinclude a substrate constructed of a material or materials havingrelatively low electrical conductivity. As described above, differentaircraft components require different degrees of lightning strikeprotection. Though a lightning strike protection surface film for a Zone1A component may require an electrically conductive substrate, anon-conductive substrate with grown-on CNTs may provide a conductivepath that is sufficient to adequately dissipate lower levels ofelectrical energy for a Zone 2A, Zone 2B or Zone 3 aircraft component orfor a non-aircraft component such as a wind turbine blade, for example.In such applications, the substrate may include a woven or braidedfiberglass fabric, for example, which can be less costly than theelectrically conductive substrates described above.

An additional embodiment includes a woven or expanded metal screen asthe substrate. The woven or expanded metal screen can be constructed ofcopper, aluminum, bronze, or the like. In one embodiment, the metalscreen can have an areal weight density of about 40 gsm to about 400gsm. A plurality of CNTs can be grown on the substrate as describedabove. In one embodiment, the grown-on CNTs can have an areal weightdensity of about 2 gsm to about 100 gsm. The combined areal weightdensities of the woven or expanded metal screen and the grown-on CNTscan be less than about 500 gsm. When used for Zone 1A lightning strikeprotection, the preform comprising the woven or expanded metal screenand the grown-on CNTs can have a surface resistivity less than about 0.5ohm/square when measured by the four-point probe method. When used forZone 2A lightning strike protection, for example, the preform can have asurface resistivity less than about 1 ohm/square.

A preform comprising a substrate with grown-on CNTs as described abovecan be infused or impregnated with a thermoplastic or epoxy resin by hotmelt or solution coating using methods known in the art, and can be laidup with the composite structure into which the substrate is beingincorporated using known methods. Known liquid molding techniques suchas resin film infusion, resin transfer molding, and vacuum assistedresin transfer molding can also be used to incorporate the preform intoan external surface portion of a composite structure. The thermoplasticor epoxy resin occupies voids within the substrate and between thegrown-on CNTs of the preform. The preform can be used as a singlesurface layer, or two or more preform layers can be incorporated into acomposite structure in overlapping layers located on or proximate to thesurface of the structure. In order to provide a smooth and aerodynamicouter surface and to minimize the possibility of surface microcracking,a polymeric material such as a polymeric adhesive can be applied overthe lightning strike protection surface film. In addition, a layer ofpaint of a type commonly used for aircraft and aircraft components canbe applied over the external surface of the structure without impedingthe ability of the preform to adequately distribute and dissipateelectrical energy resulting from a lightning strike.

When the substrate is constructed of a non-metal material such as carbonfibers, CNT-reinforced PAN carbonized fibers, CNTs, or fibers formedfrom CNTs, the lightning protection surface film can have a coefficientof thermal expansion (CTE) that is of the same order of magnitude as thein-plane CTE of the materials forming the associated compositestructure. In one embodiment, the in-plane CTE of the surface film iswithin about 90 percent to about 100 percent of the in-plane CTE of thematerials forming the associated composite structure. Accordingly, sucha lightning strike protection surface film is substantially less likelyto be prone to surface microcracking of a protected surface due tocyclic variations in temperature than a metal foil or screen having asubstantially higher CTE. Even if the total areal weight density of thesubstrate and grown-on carbon nanotubes is equal to or only slightlyless than a conventional metal foil or screen, a substantial benefit canbe realized by minimizing the possibility of microcracking arising fromthe differential thermal expansion. For example, an aircraft componentequipped with a lightning strike protection surface film according tothe invention preferably is capable of withstanding about 2,000 thermalcycles between −65° F. and 160° F. without substantial microcracking ofthe component's external surface.

When the substrate is a woven or expanded metal screen constructed ofcopper, aluminum, bronze, or the like and treated with grown-on CNTs asdescribed above, it can have a mass substantially less than metalscreens of the type presently used in prior art lightning strikeapplications and still be capable of providing equivalent lightningstrike protection. A lighter density metal screen substrate withgrown-on carbon CNTs is likely to induce substantially lower stresses inadjoining composite materials due to differences in thermal expansionthan a weightier metal screen, and thus, is less likely to be prone tosurface microcracking due to differential thermal expansion than heavierdensity prior art metal screen systems.

In one embodiment, a film as described above can replace one or morestructural prepreg plies in a laminated composite structure. In such anarrangement, the film can provide both lightning strike protection andstructural reinforcement for the composite structure, or can be usedonly for structural reinforcement. Such a composite structure caninclude one or more layers of such films, and/or can include one or morefilm layers that are thicker and heavier than the surface filmsspecifically described above. When used as a structural reinforcementlayer in a composite structure, such a film may include a substrate withgrown-on CNTs that has an areal weight density and/or a surfaceresistivity that exceeds the specific limits for areal weight densityand/or surface resistivity describe above for other embodiments.Alternatively, a film as described above could be used for all plies ina laminated composite structure. In this embodiment, significant weightsavings can be realized since the component does not need a separatelightning strike protection surface film.

In one embodiment, a fiber reinforced composite component with desirableproperties can include one or more layers comprising carbon fibers in aresin matrix. The carbon fibers have a length greater than at leastabout 6 mm and include carbon nanotubes grown on the fibers as describedabove. The nanotubes are substantially uniformly distributed on thefibers and have a length between about 1 and 100 microns. Preferably,the layer(s) of carbon fibers with grown on carbon nanotubes is disposednear the outer surface of the composite component, most preferablywithin about 0.5 mm of the outer surface of the component. Mostpreferably, the carbon nanotubes are grown on the carbon fibers suchthat one nanotube is within about 5 microns of another nanotube for atleast about 75 percent of the total number of nanotubes. Depending onthe desired material characteristics of the component, it can compriseonly one layer of carbon fibers with grown on nanotubes located on orproximate the external surface of the composite component, or the entirecomponent can be fabricated from carbon fibers with grown on nanotubes,or the component can include any number of such carbon fiber layersbetween such range.

The films described above can be incorporated into or attached tosubstantially any type of structure requiring lightning strikeprotection and/or structural reinforcement. For example, such films canbe incorporated into or attached to an aircraft engine nacelle,fuselage, wing or vertical tail, a helicopter rotor blade or otherhelicopter component, or components or portions of such structures.Additionally, they can be incorporated into or attached to otherstructures such as wind turbine blades and their support structures.Other uses will be apparent to those skilled in the art.

The above descriptions of various embodiments of the invention areintended to describe various aspects and features of the inventionwithout limitation. Persons of ordinary skill in the art will understandthat various changes and modifications can be made to the specificallydescribed embodiments without departing from the scope of the invention.All such changes and modifications are intended to be within the scopeof the appended claims.

What is claimed is:
 1. An aircraft component having an external surfaceand comprising: a. a lightning strike protection surface film disposedon or proximate to the external surface and including a preformcomprising: a substrate having a first areal weight density; and aplurality of spaced carbon nanotubes grown on the substrate, thenanotubes having a second areal weight density, wherein the nanotubesdecrease a surface resistance of the preform; wherein the sum of thefirst areal weight density and the second weight density is less thanabout 500 grams per square meter; and wherein the surface resistance isless than about 1 ohm/square.
 2. The aircraft component according toclaim 1 wherein the surface resistance is less than about 0.5ohm/square.
 3. The aircraft component according to claim 1 wherein thepreform is disposed within a polymeric material.
 4. The aircraftcomponent according to claim 1 wherein the substrate comprises aplurality of electrically conductive fibers, and wherein ends of thecarbon nanotubes are connected to the electrically conductive fibers. 5.The aircraft component according to claim 4 wherein the electricallyconductive fibers comprise carbon fibers or carbon nanotube reinforcedpolyacrylonitrile carbonized fibers, and wherein the first area weightdensity is between about 70 grams per square meter and about 400 gramsper square meter.
 6. The aircraft component according to claim 4 whereinthe substrate is a woven, braided or non-crimp fabric, or a non-wovenmat, veil, or scrim, or a paper.
 7. The aircraft component according toclaim 6 wherein the electrically conductive fibers are carbon nanotubes,wherein the first areal weight density is between about 15 grams persquare meter and about 200 grams per square meter, and wherein the sumof the first areal weight density and the second areal weight density isless than about 300 grams per square meter.
 8. The aircraft componentaccording to claim 6 wherein the electrically conductive fibers havelengths greater than about 6 mm.
 9. The aircraft component according toclaim 1 wherein the substrate comprises a metal screen or mesh, andwherein the first areal weight density is between about 40 grams persquare meter and about 400 grams per square meter.
 10. The aircraftcomponent according to claim 1 wherein the carbon nanotubes have lengthsfrom about 5 microns to about 100 microns.
 11. The aircraft componentaccording to claim 1 wherein the carbon nanotubes have diameters fromabout 1 nanometer to about 200 nanometers.
 12. The aircraft componentaccording to claim 1 further comprising a metallic material deposited onthe substrate or on the preform.
 13. The aircraft component according toclaim 1 wherein the external surface defines an aircraft engine nacelle,fuselage, wing or vertical tail, a helicopter rotor blade or otherhelicopter component, or components or portions thereof.
 14. A lightningstrike protection surface film comprising: a substrate having a firstareal weight density and a plurality of spaced carbon nanotubes grown onthe substrate, the nanotubes having a second areal weight density,wherein the nanotubes decrease a surface resistance of the surface film;wherein the sum of the first areal weight density and the second arealweight density is less than about 500 grams per square meter, and thenanotubes are substantially uniformly distributed on the fibers and havea length between about 1 and 100 microns.
 15. The lightning strikeprotection surface film according to claim 14 wherein the surfaceresistivity is less than about 1.0 ohm/square.
 16. The lightning strikeprotection surface film according to claim 14 wherein the substrate andnanotubes are disposed within a polymeric material.
 17. The lightningstrike protection surface film according to claim 14 wherein thesubstrate comprises a plurality of fibers comprising carbon fibers orcarbon nanotube reinforced polyacrylonitrile carbonized fibers, andwherein the first areal weight density is between about 70 grams persquare meter and about 400 grams per square meter.
 18. The lightningstrike protection surface film according to claim 14 wherein the spacedcarbon nanotubes are grown on a plurality of carbon nanotubes havinglengths greater than about 6 mm, wherein the first areal weight densityis between about 15 grams per square meter and about 200 grams persquare meter, and the sum of the first areal weight density and thesecond areal weight density is less than about 300 grams per squaremeter.
 19. The lightning strike protection surface film according toclaim 14 wherein the plurality of carbon nanotubes comprises carbonnanotubes having lengths from about 5 microns to about 100 microns andhaving diameters from about 1 nanometer to about 200 nanometers.
 20. Thelightning strike protection surface film according to claim 14 whereinthe substrate comprises a woven or expanded metal screen and wherein thefirst areal weight density is between about 70 grams per square meterand about 400 grams per square meter.
 21. A method of producing anaircraft component with lightning strike protection, the methodcomprising: providing an aircraft structure having an outer surfaceportion; forming a lightning strike protection surface film by providinga substrate having a first areal weight density and growing on thesubstrate a plurality of spaced carbon nanotubes, the nanotubes having asecond areal weight density, wherein the nanotubes decrease a surfaceresistance of the substrate; bonding the lightning strike protectionsurface film to the outer surface portion to form an external surfaceportion; and selecting the composition and areal weight density of thesubstrate and controlling the areal weight density of the carbonnanotubes such that the sum of the first areal weight density and thesecond areal weight density is less than about 500 grams per squaremeter and such that the surface resistivity is less than about 1ohm/square.
 22. The method according to claim 21 further comprising thestep of heat treating the surface film between about 600° C. and 1,800°C. for about 0.25 to 24 hours after growing nanotubes on the substrate.23. The method according to claim 21 wherein the areal weight density ofthe carbon nanotubes is between about 2 grams per square meter and about100 grams per square meter and wherein the substrate with grown-onnanotubes has a surface resistivity less than about 0.5 ohm/square. 24.A composite aircraft component comprising: a polymeric material; atleast one layer comprising a woven or non-woven substrate and aplurality of carbon nanotubes grown on the substrate, wherein at least75 percent of the plurality of carbon nanotubes are located within 5microns of another nanotube in the plurality of nanotubes, such that theplurality of carbon nanotubes provide a conductive path through thepolymeric material; and wherein the substrate provides structuralreinforcement to the polymeric material and wherein the carbon nanotubesprovide lightning strike protection to an outer surface of the compositeaircraft component; and wherein the substrate and the plurality ofcarbon nanotubes have a combined areal density that is less than about500 grams per square meter.
 25. The composite aircraft componentaccording to claim 24 wherein the component has an external surface andthe layer is positioned within at least about 0.5 mm of the surface.